Poco's here use barrel locks, and in extreme cases a larger version that sits on the side & front of the meter pan. That BIG thing has an additional barrel lock on it.
As to tampering, the last one I stumbled on was an apt super who would flip his meter for a few days/weeks, then flip it back prior to meter reading day. Don't know what they did to him, besides the shut-off when the wiring insp showed up to unlock a meter for me.
